Free-Electron Laser Program

Breakthrough at Jefferson Lab: Terahertz Light ("T-rays")

Spot the knife? Millimeter waves, close to terahertz, show their ability to see through
clothes and paper. Science Vol. 297, 2 Aug. 2002.

Nontechnical Summary

An experiment at the U.S. Department of Energy's Thomas Jefferson National Accelerator Facility in Newport News, Va., has shown how to make highly useful terahertz light, or T-rays, tens of thousands of times brighter than ever before. (View articles in the popular and scientific press.)

This latest spinoff from Jefferson Lab's main mission of nuclear physics research lights the way toward a number of prospective benefits, including:

better detection of concealed weapons, hidden explosives and land mines,

improved medical imaging and more productive study of cell dynamics and genes,

instant "fingerprinting" of chemical and biological terror materials in envelopes, packages or air,

better characterization of semiconductors, and

widening the frequency bands available for wireless communication.

Researchers from Jefferson Lab and two other Department of Energy laboratories - Brookhaven National Laboratory in Upton, N.Y., and Lawrence Berkeley National Laboratory in Berkeley, Calif. - sent a beam of electrons at nearly the speed of light through a magnetic field. That caused the electrons to radiate T-rays at a trillion cycles per second—the terahertz frequency that gives T-rays their name and that makes them especially useful for investigating biological molecules.

Invisible T-rays bear comparison with radio waves, microwaves, infrared light and X-rays. But unlike those much-used forms of radiated energy, T-rays have been little exploited—in part because no one knew how to make them bright enough.

T-rays are electromagnetic radiation of the safe, non-ionizing kind. They can pass through clothing, paper, cardboard, wood, masonry, plastic and ceramics. They can penetrate fog and clouds. Their wavelength—shorter than microwaves, longer than infrared—corresponds revealingly with biomolecular vibrations.

For over a decade, scientists worldwide have been pressing the study of terahertz light and looking for better ways to generate and use it. An Aug. 16, 2002, Science magazine article, "Revealing the Invisible," reported that "much research is being directed toward the development of T-ray sources and detectors, particularly for applications in medical imaging and security scanning systems." The Web site of Dr. Xi-Cheng Zhang, a T-ray expert at Rensselaer Polytechnic Institute, predicts that "the future 'killer application' ... will be in biomedicine."

Tochigi Nikon Corporation and Teraview—a Cambridge, England, start-up associated with Toshiba—have begun commercializing low-power T-ray systems. A few hospitals are testing comparatively dim T-rays for detecting skin cancer. Dr. Daniel M. Mittleman of Rice University says that for low-power T-rays, "perhaps the most promising applications lie in the area of quality control of packaged goods." He illustrates by showing how T-rays can check the raisin count in boxes of raisin bran.

Overall, though, T-rays still constitute a gap in the science of light and energy. They inhabit a region of the electromagnetic spectrum remaining to be better understood—and much better exploited. Now that a way to generate them at high power has been demonstrated, T-rays can potentially extend and add widely to the wave-based technologies that have defined the last century and a half, from the telegraph, radio and X-rays to computers, cell phones and medical MRIs.

Jefferson Lab's Gwyn Williams conceived and led the experiment, which took place during 2001. Dr. Williams and his colleagues announced the work in the article "High-power terahertz radiation from relativistic electrons" in the Nov. 14, 2002, issue of the international science journal Nature. (View this and other articles in the popular and scientific press.)

Dr. Williams is a photon scientist, a researcher of light. But Jefferson Lab's main mission is nuclear physics, the study of the atom's core. Nuclear physicists visiting from across the country and around the world probe nuclei with beams of electrons from Jefferson Lab's huge superconducting accelerator. Williams joined Jefferson Lab in 2000 to conduct research with a smaller, unique adaptation of that accelerator: the world's first high-powered "free-electron laser."

The U.S. Navy funded the laser's construction to investigate the science and technology of high-power laser beams whose precise wavelength can be selected. It's over 200 times more powerful than any other wavelength-tunable laser—and the Navy is funding an upgrade to still higher power. The laser is "driven" by the same small superconducting electron accelerator that Williams and his colleagues used for their demonstration of bright T-rays.

Before that experiment, no other method of generating terahertz light had yielded more than a thousandth of a watt in power. But they extracted tens of thousands times more. "Think of a candle and then think of a floodlight," says Williams.

But no matter how bright they are, T-rays can't penetrate metal or water. So they can't be used to inspect cargo containers on arriving ships or to diagnose conditions deep inside the human body. "Nevertheless," says Williams, "the growing awareness of T-rays' usefulness is like what happened a century ago with X-rays—only T-rays will have a much wider range of applications. The task now is to develop those uses individually."

And that's exactly what he and his colleagues are about to do. Dr. Williams is planning a program of further research to develop and demonstrate practical applications of terahertz light.